Hypoxia-responsive interaction between P-TEFb, BHLHE40, and Tim8-Tim13 regulates hypoxic gene transcription

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• The P-TEFb transcriptional kinase complex is crucial for regulating gene transcription by RNA polymerase II (RNAPII).
• Identifying specific interactions that direct P-TEFb activity to hypoxia-responsive genes is essential for understanding cellular adaptation to low oxygen environments.
• Hypoxia-inducible factors (HIFs) are key regulators of the cellular response to hypoxia, but alternative regulatory mechanisms may also exist.

Purpose of the Study:

• To identify hypoxia-specific interactions that direct P-TEFb activity to hypoxia-responsive genes.
• To elucidate a novel regulatory axis controlling the transcriptional response to hypoxia.

Main Methods:

• Biochemical purification to identify interacting protein complexes.
• Chromatin association assays to confirm interactions.
• Gene silencing (siRNA) and genetic knockout (CRISPR) to assess functional impact.
• Pharmacological inhibition of HIF-2α (belzutifan).

Main Results:

• A novel, hypoxia-specific, chromatin-associated interaction was discovered involving P-TEFb subunit cyclin T1 (CCNT1), nuclear localized mitochondrial Tim8-Tim13 complexes, and the transcription factor BHLHE40.
• Disruption of the Tim8-Tim13 complex or silencing of BHLHE40 impaired the transcriptional response to acute hypoxia.
• This CCNT1/BHLHE40/Tim8-Tim13 interaction is independent of HIF, as confirmed by genetic HIF-1β knockout and pharmacological HIF-2α inhibition.
• BHLHE40 depletion compromised the proliferation of clear cell renal carcinoma cells, highlighting its role in cancer cell adaptation to hypoxia.

Conclusions:

• A partially HIF-independent regulatory axis involving Tim8-Tim13 complexes and BHLHE40 modulates P-TEFb activity in response to hypoxia.
• This pathway represents a new mechanism for controlling gene expression under low oxygen conditions.
• The findings suggest potential therapeutic targets for hypoxia-related diseases, including cancer.